Preparation and properties of γ-Fe2O3 and Y2O3 doped γ-Fe2O3 by a sol–gel process

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Abstract

Ultra-fine γ-Fe2O3 powders with average particle size of 10 nm were prepared by a complexing sol–gel process. Thermal analysis, X-ray diffraction and FT-IR were employed to study the microstructure and thermal stability. The as-prepared γ-Fe2O3 has high response to reducing gases. γ-Fe2O3 doped with Y2O3 to form a solid solution can raise its phase transition temperature to as high as 775°C and improve the resistance stability of the sensors without great change of their response and selectivity. The lifetime problem of gas sensors based on γ-Fe2O3 is expected to be basically solved by doping method.

Introduction

Gamma iron oxide (γ-Fe2O3) is a ferromagnetic material and is widely used as a magnetic recording medium [1]. It has been studied in detail as gas-sensitive materials 2, 3. For this application, many methods, such as oxidizing Fe3O4, dehydrating γ-FeOOH, and decompositing iron oxalate (FeC2O4·2H2O) 1, 4, 5, have been applied to prepare γ-Fe2O3. The γ-Fe2O3 prepared by these methods has relatively lower response to the gases previously tested, for example, H2, C2H5OH, CH4 and C2H4 etc. However, the response of a material is related to the preparation process, therefore new method is needed to prepare γ-Fe2O3 to further improve its gas-sensing properties. On the other hand, the application of pure γ-Fe2O3 gas sensors is limited because of its lower phase transition temperature (around 390°C prepared by the method in this work), from γ-Fe2O3 to α-Fe2O3, which causes the resistance change of the sensors, affects the stability of the sensors using γ-Fe2O3 as sensitive material. Thus, proper measures should be taken to raise its phase transition temperature in order to improve the sensors' stability. In the meantime, new techniques are also needed to prepare gas-sensing materials because their properties are greatly related to the materials' microstructure, for example, the particle size and their distribution which are determined by the preparation process. Sol–gel synthesis is a good method to prepare ultra-fine powders and other materials that have particular properties have resulted in wide interests 5, 6. Generally, three types of sol–gel processes, the colloidal, inorganic polymeric and complexing sol–gel processes, are applied in experiments 7, 8. In the complexing sol–gel process, glycol is usually used as complexing agent, which has been applied to the preparation of NiO film [9]and mixed-cation oxide powders [10]. We have tried to employ sol–gel processes to prepare materials with novel gas-sensing properties [11]. However, to our best knowledge, the report on preparation of γ-Fe2O3 by sol–gel process is scarce.

In this paper, nanocrystalline γ-Fe2O3 with average particle size of 10 nm was successfully prepared by a complexing sol–gel process. The gas-sensing properties of pure γ-Fe2O3 and Y2O3 doped γ-Fe2O3 are presented. The effects of doped Y2O3 to the phase transition temperature and gas-sensing properties are also determined.

Section snippets

Experimental

Pure γ-Fe2O3 prepared by sol–gel process is rarely reported. In our work, hydrated ferric nitrate (Fe(NO3)3·9H2O) and glycol were chosen to form the precursor solutions. Twenty grams of Fe(NO3)3·9H2O were dissolved in 80 ml of glycol. The solution was well stirred, and nanocrystalline γ-Fe2O3 was finally obtained through a series of steps as shown in Fig. 1.

The molar ratio of Fe(NO3)3·9H2O to glycol is an important factor to obtain relatively pure γ-Fe2O3. We define R as the molar ratio of Fe(NO

Results and discussion

Fig. 2(a) shows the TG-DTA curves of undoped xerogel powders, measured under the following conditions: sample weight 11.13 mg, heating rate 10°C/min, flowing air rate 90 ml/min. Weight losses occurring before 135°C and between 135–200°C correspond to the desorption of water and loss of HNO3 etc., which appear on the DTA curve as two endothermic peaks at 80°C and 140°C, respectively. The exothermic peak at 250°C on the DTA is attributed to the decomposition of organic components accompanied by a

Conclusion

Ferric nitrate (Fe(NO3)3·9H2O) and glycol were used as precursors to prepare nanocrystalline γ-Fe2O3 by a sol–gel process. In order to obtain pure γ-Fe2O3 by the sol–gel process, the molar ratio of Fe(NO3)3·9H2O to glycol must be controlled in the range 1/36–1/15 and the refluxing time should be longer than 12 h at 70°C. Calcined at 300°C for 2 h, γ-Fe2O3 powders with average particle size of 10 nm were obtained. The phase transition temperature of as-prepared γ-Fe2O3 is around 390°C. The

Acknowledgements

The authors would like to thank the Chinese Natural Science Foundation for its financial support.

References (15)

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